Vehicles such as cars are equipped with seat belts for protecting the occupants from unexpected events such as an emergency stop and collision. Among such seat belts, three-point seat belts are extensively used because they are easy to wear and provide a relatively large restraining force.
The restraining device disclosed in Unexamined Japanese Patent Application Kokai Publication No. 2004-17765 comprises a retractor winding up the belt for restraining an occupant. The retractor is provided with an electric device for adjusting the tension of the belt according to the force to apply to the occupant.
The aforementioned electric device controls the retractor in the manner such that the belt has the tension of a first target value while the vehicle is normally running. Then, the electric device controls the retractor so that the belt has the tension of a second target value greater than the first target value in time of need because a large force is applied to the occupant such as when the vehicle makes an emergency stop or a turn. In such a case, the electric device gradually increases the tension of the belt from the first target value to the second target value. In this way, it is possible to restrain the occupant with a proper force without giving any discomfort.
The restraining device disclosed in the Unexamined Japanese Patent Application Kokai Publication No. 2004-17765 controls the power supply in the manner that the motor current has a target value according to the belt tension. Therefore, it is difficult to detect the current when the target current value is low, thus hampering accurate power supply to the motor. Furthermore, insufficient motor torque may cause the clutch operation to fail. On the other hand, when the target current value is high, a problem is that the motor generates a large quantity of heat.
For driving the motor used for the retractor (for example, a motor with a brush for a contact point), a voltage equal to or greater than the minimum operation voltage should be applied to the motor. The minimum operation voltage sometimes goes up as the motor deteriorates (for example, deterioration with time). Then, the motor operation may become unstable after the motor deteriorates to some extent.
The present invention is invented under the above circumstances and an exemplary object of the present invention is to operate a retractor accurately and reduce the motor power consumption by supplying to the motor used for the retractor the power of a level appropriate to the situation.